There are many industrial, medical, and other applications where high resolution (generally less than 10 micrometer) measurement of distances, thicknesses, and optical properties of a biological or other sample are required. These applications include measurements of biological tissue layers, semiconductors and other applications involving multiple thin layers of material, as well as in the non-destuctive testing of small structures such as integrated optical circuits, optical connectors, optical couplers, semiconductor lasers and semiconductor optical amplifiers. Such applications also include various medical applications including laser microsurgery and diagnostic instrumentation.
Existing techniques for performing such measurements include optical coherence domain reflectometers (OCDR), optical time domain reflectomerry (OTDR), ultrasound, scanning laser microscopes, scanning confocal microscopes, scanning laser ophthalmoscopes and optical triangulation. Existing OCDR systems do not normally have the rapid data acquisition rate required for the measurement of biological or other samples having the potential for dynamic movement; while OTDR systems are very expensive and have only limited resolution and dynamic range.
Ultrasound, which is perhaps the most commonly used technique, is disadvantageous for applications such as taking measurements on the eye in that, in order to achieve the required acoustic impedence matches, and to thus avoid beam losses and distortion, contact is generally required between the ultrasonic head or probe and the product or patient being scanned. While such contact is not a problem when scans are being performed on, for example, a patient's chest, such probes can cause severe discomfort to a patient when used for taking eye measurements such as those used for measuring intraocular distances for computing the power of lens implants.
The relatively long wavelengths employed in ultrasound also limit spatial resolution. Further, ultrasound depends on varying ultrasound reflection and absorption characteristics to differentiate and permit recording or display of tissue, or other boundaries of interest. Therefore, when the acoustic characteristics of adjacent layers to be measured are not significantly different, ultrasound may have difficulty recognizing such boundaries.
Scanning laser or confocal microscopes and scanning laser ophthalmoscopes (SLO) provide highly spatially resolved images, for example being able to generate real time video images of the eye with a lateral resolution of a few micrometers. However, the depth resolution of SLO's quickly degrade with decreasing numerical aperture. For example, SLO measurements of the retina through the pupil aperture restrict the depth resolution to roughly 200 microns. SLO's are also expensive, costing in the range of a quarter million dollars.
Optical triangulation offers fairly high resolution, but requires parallel boundaries. Such devices also have relatively poor signal-to-noise ratios and have degraded resolution at greater depths, where numerical aperature is restricted.
A need, therefore, exists for an improved method and apparatus for performing high resolution measurements and in particular for optically performing such measurements, which improved technique does not require contact with the body being measured, which maintains substantially constant high resolution over a scanning depth of interest, regardless of available aperture size and which is relatively compact and inexpensive to manufacture. Such a system should also be capable of providing differentiation between sample layers, should be able to provide identification of layer material or of selected properties thereof, should be able to provide one, two and three-dimensional images of a scanned body and should be rapid enough for use in biological and other applications where the sample being measured changes over relatively short time intervals. Finally, it would be desirable if such technique could also provide information concerning the birefringence property and spectral properties of the sample.